Two test masses per spacecraft configuration is the main scheme and exhibits a high level of maturity for space-based gravitational wave detection. However, the single test mass configuration is continuously discussed with different designs to reduce technical complexities such as spacecraft control. This paper proposes a new configuration with a single quasi-cubic test mass per spacecraft. The new configuration inherits the existing capacitance displacement sensor and electrostatic actuator. It has the advantage of the suppression of centrifugal force effect due to the coincidence of the center of mass between the spacecraft and test mass. In addition, it has the advantages of simplifying the self-gravity compensation strategy and the drag-free attitude and orbit control strategy due to using a single test mass.